Man walks with 50 kg stones

A 55-year-old man, surnamed Huang, can surprisingly lift 50kg of stones with his feet and carry them, along with two dumbbells in his hands for 200 meters in a village in Chengdu, Sichuan province. Huang said that he's walked with stones on his feet every day for years as a way to exercise and he's "worn out" seven pairs of stones.

How many pairs of stones has been worn out by the man?

A. 6

B. 7

C. 8